The state of the art recognizes processes for depositing materials composed of individual elements (metals or the like), alloys, reaction products of one or more elements and other materials including oxides, and reaction products with nitrogen by sputtering techniques utilizing single-component or multi-component targets with the target being connected cathodically. Such processes are known as cathodic sputtering or deposition and generally the substrate to be coated is connected as the anode relative to the more negative cathode or target. Such processes may involve supporting magnetic fields, in which case the cathode and target can form a magnetron, the process then being referred to as magnetron sputtering.
Magnetron sputtering is usually carried out at pressures below a value of 1.times.10.sup.-2 mBar. At these sputtering pressures it is found that the free path lengths are comparatively large so that the particles which have been atomized form the target, depending upon the target-substrate distance, scarcely interact with one another before they arrive at the substrate. As a result, by contrast with other deposition processes, the energies of these particles can be comparatively high and the process can be used to produce high quality dense coatings which are strongly adherent to the substrate and have comparatively high hardness. In this way, magnetron sputtering can be distinguished from vapor deposition processes in which, because the particles are in the vapor phase, they are at comparatively lower energies.
In recent years efforts have been made to apply such coating methods to thin film technologies and particularly there is a desire or need, especially in the production of complex high temperature superconductors (HTS) or certain oxidic ceramics which may be used for other purposes and other coatings which may have to be applied to sensitive organic carriers, to utilize particle energies which are higher than those which hitherto have been utilized for vapor deposition methods but are lower than the typical energies achieved by sputtering.
In these cases efforts have been made to use sputtering processes which have been modified by so-called thermalization. In these sputtering systems the pressure is raised somewhat so that there is reduction of the free path length of the atomized particle as a result of interparticle collisions which have a braking effect so that the particles arrive with reduced energy at the substrate.
When attempts are made to utilize such thermalization control for the particle energy with conventional magnetron sputtering, it is found that:
(i) increased pressure results in a modification of the plasma. Thus the dark space region at the cathode side is reduced significantly more strongly at the cathode surface. When such assemblies utilize standard diaphragms which are conventionally positioned to prevent ion bombardment in edge regions of the target, there is an undesirable sputtering between the cathode and the diaphragm, i.e. a substantial plasma formation between the cathode or target and the diaphragm. The formation of a plasma between the target or cathode and the diaphragm can give rise to this undesired sputtering. The problem cannot be solved by omitting the diaphragms.
(ii) For most sputtering depositions with which the invention is concerned, an abnormal sputtering range is utilized, as is the case for example in the deposition of HTS material. To achieve sputtering modes which are successful for this purpose, the high pressure range used is associated with very high energies which can give rise to premature breakdowns. In the case of depositing HTS materials this in practice has been found to directly damage the target.
(iii) The high pressure utilized in sputtering does effectively serve to thermalize the particle, i.e. cause interparticle collisions. The higher pressure however alters the sputtering conditions so that the homogeneity of the deposition decreases with pressure increase as does the uniformity of the deposition rate.